Pressure sensor and method for calibrating pressure sensors

Abstract

A precision pressure sensor in the form of a load cell has a mounting ring closed at one end by an end wall which has axially outwardly facing dead-end holes therein to form sensor beams each having an axially outwardly facing plane surface to which strain gages are bonded. The dead-end holes have bottoms forming sensor membranes. Opposite the closed end the mounting ring encloses a cavity forming a sensor space which is closed by a closure plug that simultaneously has a bore for admitting fluid under pressure to be measured into the sensor space. A ring groove in the end wall has approximately radially sloping surfaces which together with the dead-end holes determine the minimum cross-section of the measuring beams. The ring groove also determines the minimum thickness of the sensor membranes.

What we claim is: 
     
       1. A pressure sensing load cell, comprising a mounting ring having a central axis, an end wall closing one end of said mounting ring, said end wall having a plane surface facing in an axially outward direction, a plurality of dead-end holes in said plane surfaced end wall, said dead-end holes opening in said axially outward direction, sensor membrane means formed in said end wall by said dead-end holes, an axial central cavity in said mounting ring, said central cavity opening in a direction opposite said plurality of dead-end holes, substantially radially extending sensor beams formed between neighboring dead-end holes, each sensor beam having a plane axially outwardly facing beam surface extending perpendicularly to said central axis, strain gage means bonded to said axially outwardly facing plane beam surfaces of said sensor beams, each of said sensor beams having two approximately radially sloping surface portions facing said central cavity opposite said plane beam surface, a closure plug for closing said central cavity to form a sensor chamber for receiving fluid under pressure to be measured, and fluid conduit means for admitting fluid under pressure into said sensor chamber, whereby said approximately radially sloping surface portions of each sensor beam face into said sensor chamber. 
     
     
       2. The load cell of claim 1, wherein said closure plug has a central axial bore forming said fluid conduit means for admitting fluid under pressure into said sensor space. 
     
     
       3. The load cell of claim 1, wherein said sloping surface portions of said sensor beams form a groove concentrically around said central axis, said groove facing substantially axially into said sensor space, said groove passing through said sensor beams and through said membrane means for forming a membrane ring section having the smallest membrane thickness. 
     
     
       4. The load cell of claim 3, wherein said groove has a groove diameter which is substantially the same as a diameter of a circle around said central axis and passing through centers of said dead-end holes, whereby said groove has a ridge passing centrally below said membrane means. 
     
     
       5. The load cell of claim 1, wherein said sensor beams have a smallest cross-section which is determined by a diameter of said dead-end holes and by a depth of a groove formed by said sloping surface portions facing said central cavity. 
     
     
       6. The load cell of claim 1, wherein said closure plug comprises a central bore forming said fluid conduit means into said sensor space, first connector means as part of said closure plug for connecting said load cell to a source of pressure through said fluid conduit means in said closure plug, external surfaces on said closure plug for applying a tool to said closure plug, and a ring groove between said external surfaces and a closure plug end secured to said mounting ring, said ring groove having a sufficient depth for isolating said sensor beams from mounting influences. 
     
     
       7. The load cell of claim 6, further comprising second connector means at said closure plug end for securing said closure plug end to said mounting ring in said central cavity. 
     
     
       8. The load cell of claim 1, wherein said bending beams have an axial minimum height b, wherein said membrane means have a minimum axial thickness t, and wherein a ratio of t/b is within the range of 1/2 to 1/10. 
     
     
       9. The load cell of claim 7, wherein said ratio t/b is 1/6. 
     
     
       10. The use of the load cell of claim 1, as a calibration reference for calibrating pressure sensors. 
     
     
       11. A pressure sensing load cell, comprising a mounting ring having a central axis, an end wall closing one end of said mounting ring, said end wall having a plane surface facing in an axially outward direction, a plurality of dead-end holes in said plane surfaced end wall, said dead-end holes opening in said axially outward direction, sensor membrane means formed in said end wall by said dead-end holes, an axial central cavity in said mounting ring, said central cavity opening in a direction opposite said plurality of dead-end holes, substantially radially extending sensor beams formed between neighboring dead-end holes, each sensor beam having a first plane axially outwardly facing beam surface extending perpendicularly to said central axis, a second plane axially inwardly facing beam surface extending in parallel to said first plane axially outwardly beam surface, strain gage means bonded to said first plane axially outwardly facing beam surfaces of said sensor beams, each of said sensor beams further having two lateral curved surfaces formed by said dead-end holes, a closure plug for closing said central cavity to form a sensor chamber for receiving fluid under pressure to be measured, and fluid conduit means for admitting fluid under pressure into said sensor chamber, whereby said membrane means are exposed to fluid under pressure on a side facing in a direction opposite to said axially outward direction and thus opposite said strain gage means for protecting said strain gage means against said fluid under pressure. 
     
     
       12. The load cell of claim 11, wherein said closure plug has a central axial bore forming said fluid conduit means for admitting fluid under pressure into said sensor space. 
     
     
       13. The load cell of claim 11, wherein said closure plug comprises a central bore forming said fluid conduit means into said sensor space, first connector means as part of said closure plug for connecting said load cell to a source of pressure through said fluid conduit means in said closure plug, external surfaces on said closure plug for applying a tool to said closure plug, and a ring groove between said external surfaces and a closure plug end secured to said mounting ring, said ring groove having a sufficient depth for isolating said sensor beams from mounting influences. 
     
     
       14. The load cell of claim 11, wherein said bending beams have an axial minimum height b, wherein said membrane means have a minimum axial thickness t, and wherein a ratio of t/b is within the range of 1/2 to 1/10. 
     
     
       15. The load cell of claim 14, wherein said ratio t/b is 1/6. 
     
     
       16. A pressure sensing load cell, comprising a mounting ring having a central axis, an end wall closing one end of said mounting ring, said end wall having an outer rim portion and a central hub portion, both portions facing in an axially outward direction, a plurality of dead-end holes in said end wall, said dead-end holes opening in said axially outward direction, sensor membrane means formed in said end wall by said dead-end holes, an axial central cavity in said mounting ring, said central cavity opening in a direction opposite said plurality of dead-end holes, substantially radially extending sensor beams formed between neighboring dead-end holes, each sensor beam having two sloping axially outwardly facing first beam surfaces and two sloping axially inwardly facing second beam surfaces, strain gage means bonded to said axially outwardly facing slanted first beam surfaces of said sensor beams, said second sloping beam surfaces facing said central cavity, a closure plug for closing said central cavity to form a sensor chamber for receiving fluid under pressure to be measured, and fluid conduit means for admitting fluid under pressure into said sensor chamber. 
     
     
       17. The load cell of claim 16, wherein said closure plug has a central axial bore forming said fluid conduit for admitting fluid under pressure into said sensor chamber. 
     
     
       18. The load cell of claim 16, wherein said sloping axially outwardly facing first beam surfaces of said sensor beams comprise surface pairs (20, 21), each surface pair including a sector-type radially inwardly sloping surface area (20) and a spoke-type radially outwardly sloping surface area (21), and wherein said sloping axially inwardly facing second beam surfaces comprise a radially outwardly sloping ring surface (22) and a radially inwardly sloping ring surface (23) to form a ring groove concentrically around said central axis, said ring groove facing substantially axially into said sensor chamber, said ring groove passing through said sensor beams for restricting the beam cross-sectional area and through said membrane means for forming a membrane ring section having the smallest membrane thickness. 
     
     
       19. The load cell of claim 18, wherein said ring groove has a groove bottom diameter which is substantially the same as a diameter of a circle around said central axis and passing through zones where said surface areas of said surface pairs meet, whereby said ring groove has a ridge passing centrally below said zones. 
     
     
       20. The load cell of claim 19, wherein said sensor beams have a smallest cross-section where said zones are located. 
     
     
       21. A differential pressure sensing load cell, comprising a central sensor body having a mounting ring and a central axis, an end wall closing one end of said mounting ring, said end wall having a surface facing axially in one direction, a plurality of dead-end holes in said end wall, said dead-end holes opening in said one axial direction, sensor membrane means formed in said end wall by said dead-end holes, an axial central cavity in said mounting ring, said central cavity opening in an axial direction opposite said plurality of dead-end holes, substantially radially extending sensor beams formed between neighboring dead-end holes, each sensor beam having a strain gage supporting beam surface, strain gage means bonded to said strain gage supporting beam surface, each of said sensor beams further having at least two approximately radially sloping surface portions facing said central cavity, a first closure member for closing said central cavity forming a first sensor chamber for receiving a first fluid under pressure to be measured, first fluid conduit means for admitting fluid under pressure into said first sensor chamber, a second closure member secured to said sensor body for forming a second sensor chamber facing said dead-end holes for receiving a second fluid under pressure, and second fluid conduit means for admitting fluid under pressure into said second sensor chamber. 
     
     
       22. The load cell of claim 21, wherein said sensor body, said first closure member and said second closure member are axially aligned with each other, and wherein said first and second fluid conduit means are axial bores in said first and second closure members. 
     
     
       23. The load cell of claim 22, wherein said first closure member is a closure plug having a plug portion fitting into said axial central cavity of said mounting ring of said sensor body, wherein said second closure member has a central end chamber, said mounting ring of said sensor body fitting into said central end chamber, and means for securing an outer circumferential wall of said mounting ring to said second closure member and further means for securing said mounting ring to said plug portion of said first closure member. 
     
     
       24. The load cell of claim 21, wherein each of said closure members comprises a respective isolating ring groove for substantially preventing external influences from adversely affecting a measured result.